The present invention relates to a broadcasting signal receiving apparatus. More particularly, it relates to a broadcasting signal direct-receiving circuit which generates a signal having the same frequency as that of a received radio frequency (RF) signal and then mixes the generated signal with the received RF signal to output a baseband signal.
In a conventional broadcasting signal receiving apparatus such as a television set or video tape recorder, the RF signal received by the apparatus is converted into an intermediate frequency IF signal which is then converted into a baseband signal in a demodulator so as to be provided to other devices in the apparatus.
The conventional broadcasting signal receiving circuit as shown in FIG. 1 is a broadcasting signal receiving circuit of a heterodyne reception system in which an IF is used in converting the received RF signal into a baseband signal. In FIG. 1, an input tuner 10 consisting of a band-pass filter is tuned to a wide bandwidth and receives a wide band RF signal among radio waves which are transmitted, the band-pass filter having its frequency bandwidth determined by the frequency of the signal generated by a phase-locked loop portion (hereinafter referred to as PLL).
The received wide band RF signal is amplified to a specific gain in an RF amplifier 20 according to the control signal AGC supplied from an automatic gain controller (not shown).
Thereafter, the RF signal amplified in the RF amplifier 20 is received as a high frequency signal corresponding to the user's desired channel by an interstage tuner 30 in which a frequency band to be tuned is varied according to the frequency of the signal generated by the PLL 40.
Thereafter, the RF signal output from the interstage tuner 30 is mixed in a mixer 60 with a signal of a specific frequency which is generated by a local oscillator 50 under the control of the PLL 40, so as to be an IF signal. This IF signal is then filtered by a band-pass filter 70, amplified by an IF amplifier 80 and thereafter demodulated by a demodulator 90, to be output as a baseband signal.
However, such a conventional broadcasting signal receiving circuit of a heterodyne reception system has a noise problem such as a phenomenon wherein many pictures overlap each other when the received broadcasting signals are displayed on the screen, due to a mutual interference of an image frequency component signal generated by the noise component included in the RF signal input to the mixer 60 and the IF signal during output or the IF signal from the mixer 60. Also, the conventional receiving circuit requires the demodulator 90 in order to convert the IF signal output from the mixer 60 into a baseband signal, and accordingly circuit construction becomes complicated, resulting in difficulties in making the receiving circuit compact and light.